It's similar to "may..., but" as in "I may be stupid, but I know what love is." The "may" in this sentence doesn't mean "It is possible that..." or "I am not certain, but there is a possibility that..." It's more like "I know I'm stupid, but..." The かもしれない part is pretty much the same as this "may," and that's why your example sentences have が and でも, which are often translated as "but" or "however."

"Certainly XXX, but..." also has a contrasting usage similar to the may+but in English. In Japanese, you can combine the may+but combo and the certainly+but structure. I think the 確かに version has a stronger sense that you are accepting the truth of the first clause than the 確かに-less version, and probably that's why 確かに is often added when the second clause sharply contradicts the listener's opinion.

IceCream wrote:

Does it make it more polite?

I don't know if it's called polite, but overusing 確かに...かもしれないが can make you sound like a pseudo-intellectual, especially when you throw in big words.